1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Do Tire Pressures effect the speedo/odo reading?

Discussion in 'Gen 2 Prius Main Forum' started by uart, Sep 13, 2009.

  1. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,215
    1,201
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    I know that a lot of people here run their tires a few PSI over the recommended levels. I was just wondering if anyone had ever tested whether or not this changes the rolling circumference and hence the accuracy of the speedometer and odometer readings?
     
    #1 uart, Sep 13, 2009
  2. CharlesJ

    CharlesJ Member

    Joined:
    Dec 13, 2008
    467
    29
    38
    Location:
    Monterey
    Vehicle:
    2008 Prius
    I seriously doubt you would read a difference. The circumference changes so little, hardly worth mentioning.
     
    #2 CharlesJ, Sep 14, 2009
  3. TKY

    TKY Member

    Joined:
    Jul 7, 2009
    232
    25
    0
    Location:
    Colorado
    Vehicle:
    2010 Prius
    Model:
    IV
    Uart, I agree with Charles; the change in circumference based on two, three, four, or five pound air pressure would, in the most likely, not register as a change in circumference. Besides that, I have driven through a couple of stationary radar display stations, several different times. My dash reading is consistently two MHP faster than the readout returned on the radar unit.

    That seems like a worse error percentage than I would get by adding a few pounds of pressure. --TK
     
    #3 TKY, Sep 14, 2009
  4. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,215
    1,201
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    Yeah I agree that you wouldn't expect much change. I was just inflating my tires yesterday (to 42/40) and was pondering why higher pressure gives lower rolling resistance. I figured it must be because the tire deflects less, so less deformation gives fewer losses, but also a smaller contact patch and perhaps a very slightly larger roll out distance. Anyway I thought it would probably be negligible but was just wondering if anyone had ever tried measuring it. (I figured if anyone on Earth had gone to the bother of testing it then it would probably be someone here, hehe :D).
     
    #4 uart, Sep 14, 2009
  5. DetPrius

    DetPrius Active Member

    Joined:
    Apr 24, 2009
    545
    92
    0
    Location:
    Southeast Michigan
    Vehicle:
    2010 Prius
    Model:
    III
    I don't think so. I'm running 42/40 and my GPS reports 1 MPH under the speedo.
    EDIT: This is at highway speeds - 55 to 70. I haven't looked at it at lower speeds.
     
    #5 DetPrius, Sep 14, 2009
  6. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,215
    1,201
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    Ok so both yourself and TKY have reported that your Prius speedo's read slightly fast. That means that even if there where any small increase in the roll out distance, which would make the speedo read just a little slower, then it would probably make it more accurate (since it's currently reading a little over speed). That's good to know.
     
    #6 uart, Sep 14, 2009
  7. PriusLewis

    PriusLewis Management Scientist

    Joined:
    Feb 25, 2009
    1,002
    84
    7
    Location:
    Denver Metro
    Vehicle:
    Other Non-Hybrid
    Model:
    N/A
    My opinion, too. Try pushing a car with a near flat tire, then try pushing it with the tires inflated. HUGE difference. So pressure really does reduce rolling resistance. BTW, I don't know why but my speedo is really close to right on, maybe off 1 MPH @ 70 (which would be a half MPH @ 35).
     
    #7 PriusLewis, Sep 14, 2009
  8. BAllanJ

    BAllanJ Active Member

    Joined:
    Dec 9, 2008
    667
    78
    0
    Location:
    Kingston Ontario
    Vehicle:
    2017 Prius
    Model:
    Two
    A bigger change in circumferrence would probably be seen between new and worn tires than between inflated a few psi different. IMHO, anyway.
     
    #8 BAllanJ, Sep 14, 2009
  9. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,075
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    The circumference of a radial belted tire does not change with pressure. Therefor the speedometer and odometer calibration is not affected.

    Tom
     
    #9 qbee42, Sep 14, 2009
  10. Celtic Blue

    Celtic Blue New Member

    Joined:
    Aug 4, 2008
    2,224
    139
    0
    Location:
    Midwest
    Vehicle:
    2008 Prius
    The circumference change appears to be smaller than the indicated mileage impact.

    The effective circumference should change slightly, that is an important aspect of the mpg improvement: reduced friction losses from sidewall deformation. The best way to test the circumference effect is probably to do an odometer test over a fixed distance with multiple tire pressures, including repeats in each direction.
     
    #10 Celtic Blue, Sep 15, 2009
(You must log in or sign up to post here.)